PROJECT SUMMARY Invasive fungal infections are estimated to kill one and a half million people annually, with Cryptococcus neoformans (Cn) infection resulting in almost half of all deaths due to fungal infection. Cn is rare in healthy individuals, suggesting the immune system is able to prevent disease. Yet the high mortality observed clinically suggests current antifungal drug treatments are inadequate in immunocompromised individuals. This paradox suggests we are missing critical components of the Cn-host interaction. Characterization of cryptococcal cells during infection has revealed that Cn produces a unique cell type - referred to as ?titan cells? - during infection that alter the host-pathogen interaction. These titan cells are produced in response to the host pulmonary environment and are 5-10x larger than typical-sized Cn cells. We demonstrated previously that titan cell production is critical for virulence and impacts dissemination to the CNS. We further showed titan cell formation alters the host response by reducing phagocytosis and stimulating a detrimental Th2-mediated response. The interaction between pathogens and their human hosts can be very complex, and the outcome depends on both host and pathogen responses. The host must sense pathogen associated molecular patterns (PAMPs), and then produce an appropriate immune response to kill the pathogen. Conversely, the pathogen must sense and respond to the host environment to promote its own survival. This proposal aims to identify critical alterations involved in the genesis of titan cells and their progeny, as well as define how their unique cellular structure impacts the host immune response and thereby, pathogenesis. Titan cells have a number of unique characteristics, thus the focus of our proposed investigations are to determine how these traits influence: 1) pathogen adaptation/survival in the host, and 2) alteration of PAMPs recognized by the host immune response. Our previous studies showed titan cells undergo ploidy changes associated with their formation and replication, and lead us to hypothesize these changes are critical for adaptation and survival in the host environment. Therefore, our first aim is to define cell cycle regulation needed to generate titan cells and their progeny. In our second aim, we will test the hypothesis that alternations in the titan cell wall impact PAMP recognition by the host, with the third aim determining how this altered sensing generates the detrimental immune response observed in response to titan cells. These studies will ultimately coalesce into multi-faceted antimicrobial therapies that combine targeting patho- gen-specific processes to limit pathogen adaptation and modulation of the host immune response to maximize beneficial host responses to reduce disease.